Apron feeders are typically used in the mining, cement, and other bulk materials industries for extracting bulk materials from bins, hoppers, silos, stockpiles, and the like. Apron feeders are specified for feeding wet, sticky, lumpy, large, abrasive, and heavy materials. The apron feeders are often used to control material feed rate from hoppers and to transfer material between locations. Some specific uses of apron feeders known in the art are applications such as feeding and withdrawing materials from primary crushers, loading and unloading trucks and railcars, removing frozen materials from storage, feeding jaw crushers and belt conveyors, and high abrasion applications frequently found in reclaim circuits. Early forms of apron conveyors include U.S. Pat. No. 3,934,712 to Jende and U.S. Pat. No. 1,537,444 to Herzog.
As shown in FIG. 1, typically apron feeders comprise a frame supporting an endless linked flight or pan conveyor. An upper surface or material bed of the conveyor receives material, such as from a hopper, and conveys the material a short distance to a head shaft, before the conveyor belt rolls over the head shaft and returns to the tail end to repeat the operation.
The conveyor belt comprises two or more parallel and endless conveyor chains and a series of individual and transverse-extending pans connected to the chains and which extend transversely to the chains, forming a substantially continuous material-bearing bed surface. Conveyors of more current apron feeders implement more substantial pans, each of which is mounted atop a chain link, pans or groups of pans alternating between a featureless surface and material engaging plates or grousers.
The head shaft is fit with drive sprockets for synchronously pulling the chains, connected pans and supported material to the head shaft. A tail shaft or spaced tail guide sprockets are located at an opposite end of the frame from the head shaft for receiving the returning conveyor portion and directing it back towards the head shaft. A slide, plurality of idler rollers or both are located between the head and tail for supporting the conveyor under the weight of the conveyor and its material load.
Each of the pans is independent and is closely adjacent or overlapped with the adjacent pan capable of forming a continuous and planar upper surface for bearing process material thereon, whilst remaining separate or discrete so as to enable rolling transport about the head shaft and tail sprockets. The live load of material is generally funneled by the hopper walls to the material bed of the conveyor. The hopper walls are spaced laterally apart and straddle the material bed.
The conveyor belt comprises two or more parallel and endless conveyor chains, typically two, and a series of individual and transverse-extending pans connected to the chains, forming a substantially continuous material-bearing bed surface. The pans are connected to the chains adjacent the pans opposing ends. The pans are arranged like adjacent railway ties extending transverse to spaced chains, typically two chains. The pan is fastened or connected to each chain, and forms a middle span portion between the chain connections. Each of the opposing ends of the pans projects outward, cantilevered from its respective the chain connection. The pan forms a simply supported beam, transferring load to the chain connections.
The location of the connection of the pans, to the spaced chains, whether closer to the pan's middle or closer to the pan's ends, falls into one of two configurations: inboard and outboard chain connections.
Inboard chain connection locates the chains laterally inward, closer to the pan's middle portion and within the bounds of the material bed. The hopper walls are located outward of the chain connections. This places the chain connections under the live load, with greater exposure of the chain connections to the conveyed material. Inboard chain connection minimizes bending loads between the parallel chain drive connections and, due to the nature of the material, is preferably used in the conveyance of rock. A disadvantage of placing the moving components, and particularly the chain components, and chain connections under the live load is exposure to material spillage and poor maintenance access.
Outboard chain connection places the chain's connections wider apart, outboard, or outward of the feeder walls, and is better suited for better protecting the chain's pan connections, links and drives from highly abrasive materials. Further, the chain links and connection are outside the hopper walls and apron feeder structure for ease of access, repair and component replacement. The disadvantage is an increased span of the middle portion between the chain connections resulting in higher bending loads and stresses.
Bottom edges of the spaced hopper walls terminate with a wall gap, short of the pan's upper surface, and are inward of the pan's ends to minimize material losses. To minimize spillage, each pan is equipped with a side wall, plate or wing that stands upright from each end of the pan adjacent and just outward of the hopper walls. The wings are spaced to straddle the hopper wall's bottom edges. The wings are oriented to complement the plane of the respective bottom edges and are located laterally immediately outward and adjacent thereto to minimize material escape out from the wall gap. Each wing is also angled or laterally splayed and overlapping to overlap axially with like wings on adjacent pans to reduce spillage between wings on adjacent and discrete pans, creating a substantially continuous sidewall over the entire length of the material bed of the apron feeder.
On feeders with inboard chain connections, such as U.S. Pat. No. 6,662,930 to Yester (Metso Minerals Industries Inc, Pittsburgh, Pa.), and reproduced here as FIG. 1, the wings are typically attached to the outwardly-cantilevered, distal ends of each pan. The chains are inboard of the hopper walls. The distal ends are at the lowest stress location on the pan. As the wings are secured to the free ends of the pans, the mechanical design for attachment is not a design issue of concern. The wings are typically welded to the pan's distal ends.
On feeders with outboard chain connections, attachment of the wings to the pans is made inward of the each chain's connections, between the respective chain connection and the hopper walls. This also is a location subject to the highest stress points on the pans under live loads. In one case, shown in U.S. Pat. No. 2,416,634, outboard chain drives are provided and sidewalls or wings are welded to the pans. This early design has been used to date, including in Applicant's own apron feeders with outboard chain connections. As a result, failures occur at a higher frequency than desired. Applicant notes that welding of the wings to the pans causes a heat-affected-zone (HAZ) that results in a stress concentrated zone and requiring specialty welding procedures and ultimately reduced fatigue life.
The welded wings are located inward of the outboard connection to the chain links and thus are in the high stress zone that is susceptible to failure and limits the life of the pan. Apron feeders are typically used in high volume, difficult material handing environments, the operation of which are sensitive to unexpected failures or frequent servicing. If not detected, Applicant is aware of pan failures through catastrophic separation of one end of the pan at the weld and loss or driving connection to the chain. The opposing end of the pan, which is unlikely to have failed simultaneously, continues to be driven until detection. The large horsepower of the drives results in significant damage to adjacent hardware and typically a shutdown of the entire apron feeder.
There remains an objective to use outboard chain connections for apron feeder pans for conveyance of difficult materials, yet while minimizing pan structural failures.